1. Field of the Invention
The present invention relates to a Raman amplification method, a Raman amplifier, and an optical transmission system which Raman-amplify a plurality of channels of signal light having wavelengths different from each other.
2. Related Background Art
Optical fiber amplifiers are optical devices which amplify signal light propagating through optical fiber transmission lines in optical transmission systems, in order to compensate for the transmission loss in the optical transmission lines. An optical fiber amplifier placed on an optical transmission line comprises an amplification optical fiber also functioning as an optical transmission line, and pumping light supply means for supplying pumping light to the amplification optical fiber. When fed into an amplification optical fiber supplied with pumping light, signal light is amplified in the amplification optical fiber.
Known as such an optical fiber amplifier are rare-earth-doped fiber amplifiers doped with rare earth elements such as Er (erbium), and Raman amplifiers utilizing Raman amplification phenomena caused by stimulated Raman scattering.
Here, the rare-earth-doped fiber amplifiers (e.g., EDFA: Erbium-Doped Fiber Amplifier) utilize an optical fiber doped with a rare earth element (e.g., EDF: Erbium-Doped Fiber) as an amplification optical fiber. On the other hand, the Raman amplifiers utilize a silica-type optical fiber constituting optical fiber transmission lines and the like as a Raman amplification optical fiber.
In the above-mentioned optical amplifiers, the Raman amplifiers are advantageous in that they can use a given wavelength band as an amplification wavelength band by choosing an appropriate wavelength of pumping light. If a plurality of pumping light sources respectively supplying different wavelengths of pumping light to a Raman amplification optical fiber are prepared, signal light can be amplified while using respective wavelength bands determined by the individual channel wavelengths included in the pumping light as amplification wavelength bands.
The inventors studied conventional techniques and, as a result, have found the following problems. Recently, due to social needs at the advent of highly information-oriented society, studies and developments concerning large-capacity, high-speed communications and long-haul communications have vigorously been under way. Here, a wavelength division multiplexing (WDM) transmission system carries out high-speed/large-capacity optical communications by transmitting a plurality of channels of signal light having wavelengths different from each other through an optical fiber transmission line. Also, for further increasing the capacity, the signal wavelength band of WDM signal light has been in the process of widening in the WDM transmission system.
When amplifying WDM signal light in such a WDM transmission system, it is necessary for its amplification wavelength band to become wider. In the above-mentioned Raman amplifiers, on the other hand, the amplification wavelength band obtained by a predetermined wavelength of pumping light is limited. Also, as the amplification wavelength band widens, it has been becoming difficult for an amplification wavelength band enhancing method such as one increasing the number of pumping light sources to sufficiently respond to the widening of the amplification wavelength band.
Examples of other methods proposed therefor include one comprising the steps of dividing the signal wavelength band of WDM signal light into a plurality of wavelength bands and amplifying WDM signal light in each of thus divided wavelength bands, and one amplifying WDM signal light by combining a Raman amplifier and a rare-earth-doped fiber amplifier (see, for example, Japanese Patent Application Laid-Open No. 2001-085773, Japanese Patent Application Laid-Open No. HEI 11-084440, and OFC2001, PD-24). In these methods, however, the configuration of the optical transmission system becomes complicated as a whole, thereby increasing the cost. Also, they have not fully realized signal amplification throughout the signal wavelength band.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a Raman amplification method enabling Raman amplification of WDM signal light in a simpler configuration over a wider amplification wavelength band, a Raman amplifier realizing the Raman amplification method, and an optical transmission system including the Raman amplifier.
The Raman amplification method according to the present invention Raman-amplifies a plurality of channels of signal light (WDM signal light) having wavelengths different from each other. In particular, for achieving the above-mentioned object, the Raman amplification method according to the present invention Raman-amplifies a part of the WDM signal light by supplying pumping light having a wavelength xcexp to an amplification optical fiber through which the WDM signal light propagates, and utilizes at least a part of the Raman-amplified light (part of the WDM signal light) as pumping light, so as to Raman-amplify a part of WDM signal light including a channel wavelength with a wavelength of (xcexp+xcex94xcex+20 nm) or longer, where xcex94xcex is the Raman shift amount of wavelength caused by the pumping light at the wavelength xcexp.
The Raman amplifier according to the present invention Raman-amplifies a plurality of channels of signal light (WDM signal light) having wavelengths different from each other. The Raman amplifier comprises a Raman amplification optical fiber, pumping light supply means, and multiplexing means. The pumping light supply means supplies pumping light having a wavelength xcexp to the Raman amplification optical fiber. The multiplexing means guides to the Raman amplification optical fiber the pumping light from the pumping light supply means. The Raman amplification optical fiber transmits the WDM signal light therethrough, whereas pumping light having a wavelength xcexp for Raman-amplifying at least a part of the WDM signal light is supplied there to by way of the multiplexing means. In particular, the Raman amplifier utilizes at least a part of the light Raman-amplified by the pumping light having the wavelength xcexp as pumping light, thereby Raman-amplifying a part of WDM signal light including a channel wavelength with a wavelength of (xcexp+xcex94xcex+20 nm) or longer, where xcex94xcex is the Raman shift amount of wavelength caused by the pumping light at the wavelength xcexp.
For WDM signal light in a wider signal wavelength band, the above-mentioned Raman amplification method and Raman amplifier carry out Raman amplification utilizing pumping light having a wavelength xcexp, and Raman amplification utilizing a part of the WDM signal light as pumping light. This Raman-amplifies a part of WDM signal light including a channel wavelength with a wavelength of (xcexp+xcex94xcex+20 nm) or longer, thus being able to Raman-amplify WDM signal light over a wider amplification wavelength band. Also, since it is not necessary for the WDM signal light to be divided into a plurality of wavelength bands and amplified in each wavelength band, the configuration and the like as an optical amplifier are simplified.
Specifically, it is preferred that the wavelength xcexp of pumping light in the Raman amplification method (Raman amplifier) be set so as to Raman-amplify at least first signal light including a signal channel within a first wavelength band located on the shorter wavelength side of an amplification wavelength band. On the other hand, it is preferred that the Raman-amplified first signal light be utilized as pumping light so as to Raman-amplify second signal light including a signal channel within a second wavelength band located on the longer wavelength side than is the first wavelength band. As a consequence, the whole signal wavelength band is included in a wavelength band which can be amplified (amplification wavelength band), whereby signal light can be amplified reliably and fully.
The pumping light supply means may supply a plurality of channels of pumping light having wavelengths different from each other to the amplification optical fiber as pumping light. This, together with the above-mentioned configuration for causing a part of the signal channel to function as a pumping channel, makes it possible to further widen the amplification wavelength band that can be Raman-amplified.
The pumping light traveling direction in the amplification optical fiber may be the same as the signal traveling direction. Such a copropagation-pumping configuration secures a sufficient Raman amplification gain throughout the amplification wavelength band. The pumping light traveling direction in the amplification optical fiber may be opposite from the signal traveling direction. Such a counterpropagation-pumping configuration effectively restrains signals from deteriorating due to nonlinear phenomena in the amplification optical fiber. Also, the pumping light supplied to the amplification optical fiber may include forward pumping light whose traveling direction in the amplification optical fiber is the same as the signal light traveling direction, and backward pumping light whose traveling direction in the amplification optical fiber is opposite from the signal light traveling direction. Such a bidirectional pump configuration not only secures a sufficient Raman amplification gain, but also effectively restrains signals from deteriorating due to nonlinear phenomena. The foregoing configurations of Raman amplifier are set in view of specific conditions such as signal wavelength bands of WDM signal light.
Preferably, in the Raman amplifier according to the present invention, the amplification optical fiber includes a unitary optical fiber capable of Raman-amplifying signal light. This makes the structure of the optical amplifier simpler than that in the configuration also using amplification effected by a rare-earth-doped optical fiber and the like.
The optical transmission system according to the present invention comprises an optical fiber transmission line through which a plurality of channels of signal light (WDM signal light) having wavelengths different from each other propagates; and a Raman amplifier, placed at a predetermined position on the optical fiber transmission line, having the configuration mentioned above (Raman amplifier according to the present invention). This configuration yields an optical transmission system which, even when transmitting WDM signal light in a wider signal wavelength band, can fully Raman-amplify each signal channel of the WDM signal light, thereby enabling favorable optical transmission.
In the optical transmission system, the amplification optical fiber of the Raman amplifier may include an optical fiber provided separately from the optical fiber transmission line. In this case, the Raman amplifier becomes a lumped optical amplifier. Alternatively, the amplification optical fiber of the Raman amplifier may constitute a part of the optical fiber transmission line. In this case, the Raman amplifier becomes a distributed constant optical amplifier.
The optical transmission system may comprise a first Raman amplifier (lumped Raman amplifier) having a configuration similar to that of the above-mentioned Raman amplifier (optical amplifier according to the present invention), whose amplification optical fiber is an optical fiber provided separately from the optical fiber transmission line, and a second Raman amplifier (distributed constant Raman amplifier) having a configuration similar to that of the above-mentioned Raman amplifier (Raman amplifier according to the present invention), whose amplification optical fiber is an optical fiber constituting a part of the optical fiber transmission line.
Preferably, in the optical transmission system, a predetermined signal channel in the signal channels included in the WDM signal light has a signal light power higher than the input signal light power to the optical fiber transmission line in a predetermined segment of the optical fiber transmission line. This fully Raman-amplifies the signal light throughout a wider amplification wavelength band. The input signal light power to the optical fiber transmission line may be the same among the individual signal channels of WDM signal light. This facilitates power control of each signal channel in the optical transmission system. When a channel functioning as a pumping channel in the channels included in WDM signal light is not utilized as a signal channel, the optical transmission system transmits dummy signal light having the same wavelength as that of the unutilized channel. This enables stable Raman amplification throughout the amplification wavelength band regardless of the transmission state of WDM signal light (the state of use of each signal channel in the WDM signal). Preferably, in the optical transmission system, signal channels included in the WDM signal light are successively used in order of elongating wavelength. This can favorably widen the amplification wavelength band effected by Raman amplification.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.